For effectively protecting occupants of a motor vehicle as well as other road users in case of a collision, a fast and reliable detection of an impact and a correct evaluation of the severity of an accident is necessary. The sensor technology shall recognize an impact within the shortest possible time, but shall, however, only lead to a triggering of the protection devices for occupants and other road users, if the severity of the accident is correspondingly high, since the protection devices cannot be easily reset in many cases. Shocks, curb jostles or a hammer blow or a rock fall against the outside vehicle skin at first show rather significant amplitudes, however, they may not lead to the triggering.
For this purpose, for decades the low-frequency acceleration directed against the direction of travel arising with the impact, i.e. the delay is evaluated, the integral of which essentially describes the reduction of speed in the impact. Due to the safety crumble zones of the vehicle, however, the acceleration signals in the interior of the vehicle occur clearly delayed in case of an impact. Acceleration sensors, so-called pole catchers, upfront sensors or side satellites, displaced from the interior into the edge region of the vehicle, can in fact respond much earlier for the respective impact zone, however, they act only locally and must therefore be mounted in larger number, which in turn increases the costs significantly.
Therefore, during the last years intensive research has been carried out into the use of alternative sensor concepts. In doing so, high-frequency oscillations in the vehicle body, also called solid-borne sound, are considered in the impact. If acceleration sensors are sufficiently sensitive also for high-frequency oscillations, then they can detect apart from the low-frequency acceleration signals also the superimposed high-frequency solid-borne sound signals.
The solid-borne sound results from the resulting oscillations with force applied on the body, wherein solid-borne sound reacts both on plastic as well as on elastic deformations and has a significantly faster signal propagation time and therefore allows for a much earlier impact recognition. On the other hand solid-borne sound signals occur also with conventional non-trigger cases in significant strength, so that it was difficult to reliably judge on this basis the severity of the accident.